1. Field of the Invention
The present invention relates generally to the fields of automatic gain control (AGC) in orthogonal frequency division multiplexing (OFDM) communication receivers.
2. Description of the Related Art
Orthogonal frequency division multiplexing (OFDM) is a robust technique for efficiently transmitting data over a channel. This technique uses a plurality of sub-carrier frequencies (sub-carriers) within a channel bandwidth to transmit the data. These sub-carriers are arranged for optimal bandwidth efficiency compared to more conventional transmission approaches, such as frequency division multiplexing (FDM), which waste large portions of the channel bandwidth in order to separate and isolate the sub-carrier frequency spectra and thereby avoid inter-carrier interference (ICI). By contrast, although the frequency spectra of OFDM sub-carriers overlap significantly within the OFDM channel bandwidth, OFDM nonetheless allows resolution and recovery of the information that has been modulated onto each sub-carrier. Additionally, OFDM is much less susceptible to data loss due to multipath fading than other conventional approaches for data transmission because inter-symbol interference is prevented through the use of OFDM symbols that are long in comparison to the length of the channel impulse response. Also, the coding of data onto the OFDM sub-carriers can take advantage of frequency diversity to mitigate loss due to frequency-selective fading.
A digital receiver, such as an OFDM communication receiver, will generally include a variable gain amplifier where a gain of the amplifier is adjusted by a control signal. The process of adjusting the gain of a received signal using a control signal is called automatic gain control (AGC). Typically, an AGC process in a digital receiver involves measurement of an output signal power of the variable gain amplifier. The measured value is compared with a value representing the desired signal power and an error value for the variable gain amplifier is generated. The error value is then used to control amplifier gain so as to adjust the signal strength to coincide with the desired signal power. To affect digital demodulation with an optimal signal-to-noise ratio (SNR), AGC is used to hold the magnitude of the waveforms close to the full dynamic range of the analog-to-digital converters. This generally requires, however, that AGC be provided over the full dynamic range of the received signal power. Unfortunately, in wireless environments, a digital receiver may receive a signal which experiences rapid and wide variations in signal power due to, e.g., channel fading and other causes.
Accordingly, there is an existing need to provide an automatic gain control in an OFDM receiver that can sufficiently and rapidly handle wide fluctuations in signal power due to channel fading and other causes.
Automatic gain control (AGC) methods and apparatus suitable for use in orthogonal frequency division multiplexing (OFDM) receivers are described. One AGC method includes the steps of repeatedly performing a first AGC process which adjusts amplifier gain based on determining that a signal level of multiple time sample values is outside a limit set by a first predefined threshold; and repeatedly performing a second AGC process which adjusts the amplifier gain based on determining that a signal level of multiple frequency sample values associated with a plurality of pilot tones is outside a limit set by a second predefined threshold. Preferably, the first AGC process is performed repeatedly at a first rate and the second AGC process is performed repeatedly at a second rate that is less than the first rate.